Radio frequency (RF) and microwave signal generators use a high power amplifier to generate large output signals. Unfortunately, non-ideal characteristics of these amplifiers can cause harmonic distortion of the output signal. Harmonic distortion is nonlinear distortion on the output of an amplifier characterized by the appearance in the output of harmonics other that the fundamental component when the input wave is sinusoidal. In a high-quality signal generator, these harmonics must be filtered before the signal leaves the source. The filter should reject signals an octave and greater above the fundamental output frequency. Since the frequency range of the output signal may be a decade or more, multiple sub-octave filter ranges are used. To enable the signal generator to sweep the output signal through a large range of output frequencies very quickly, or to allow computer control of the instrument, it is desirable that the switching between the multiple sub-octave filters be electronically controlled.
A p-i-n diode may be used as electronically controlled switch. (A p-i-n diode is constructed so that an intrinsic layer, the "i region," is sandwiched between a positively doped layer ("p layer") and a negatively doped layer ("n layer"). When reversed biased, a p-i-n diode behaves like an "open" switch. When forward biased, a p-i-n diode behaves like a "closed" switch.
An appropriate arrangement of p-i-n diodes is currently used to route the output signal through the appropriate sub-octave filter and then back to an output node. At the input node, multiple series p-i-n diodes are arranged in a star configuration with one common driven node and multiple intermediate nodes. A shunt diode is placed at each intermediate node. Each intermediate node is then connected to the input of one of the sub-octave filters. The reverse of this arrangement is used to route the outputs of the sub-octave filters back to a common output node. The path through the desired filter is used by forward biasing the series diodes in that path and reverse biasing the shunt diodes connected to that path. The series diodes on the unused paths are reversed biased and the shunt diodes connected to the unused paths are forward biased. The forward biased shunt diodes provide a path to AC ground, effectively stopping any AC signal from propagating down the path the diode is shunting. This isolates the unused paths.
This method of routing signals requires very compact assembly for frequencies above 20 GHz. This is difficult and expensive. Beam lead diodes are more expensive than other types of diode packaging. These diodes should be bonded directly to the shunt diodes. In a very compact assembly, this is a manual operation that increases cost. A series diode is also required to have low capacitance to achieve high isolation when shut off. Because relatively high series resistance is typically associated with low capacitance, the series diodes tend to contribute significant signal loss. Finally, due to their nonlinear characteristics, the series diodes introduce nonlinear distortion that degrades the quality of the output signal.
Accordingly, there is a need in the art for an improved way of electronically switching RF and microwave signals. It is desirable that such a system eliminate series diodes in the signal path since these devices are expensive, hard to assemble, reduce output power, and degrade the quality of the output of the device. Finally, the system should be easy to assemble.